1. Field of the Invention
The present invention relates to an IPS (in-plane switching) mode LCD (liquid crystal display), and the control of liquid crystal molecules therein.
2. Description of Prior Art
A liquid crystal display utilizes the optical and electrical anisotropy of liquid crystal molecules to produce an image. The liquid crystal molecules have a particular passive orientation when no voltage is applied thereto. However, in a driven state, the liquid crystal molecules change their orientation according to the strength and direction of the driving electric field. A polarization state of incident light changes when the light transmits through the liquid crystal molecules, due to the optical anisotropy of the liquid crystal molecules. The extent of the change depends on the orientation of the liquid crystal molecules. Thus, by properly controlling the driving electric field, an orientation of the liquid crystal molecules is changed and a desired image can be produced.
The first type of LCD developed was the TN (twisted nematic) mode LCD. Even though TN mode LCDs have been put into use in many applications, they have an inherent drawback that cannot be eliminated; namely, a very narrow viewing angle. By adding compensation films on TN mode LCDs, this problem can be ameliorated to some extent. However, the cost of the TN mode LCD is increased. Therefore, a totally different driving means called IPS (in-plane switching) was proposed as early as in 1974. Then in 1993, Hitachi Corporation filed its first US patent application concerning IPS, in which an IPS mode LCD was disclosed.
A conventional IPS mode LCD has an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The liquid crystal layer has a plurality of liquid crystal molecules which have a same orientation when not driven, this orientation being parallel to the substrates. Pixel electrodes and common electrodes are disposed on the lower substrate. When a voltage is applied to the electrodes, an electric field is generated between the electrodes. The electric field drives the liquid crystal molecules to rotate, so that they have a new orientation that is still parallel to the substrates. The change in orientation results in a change in light transmission. In other words, the operation of the IPS mode LCD is such that the liquid crystal molecules rotate in a plane parallel with the substrates in order to fulfill optical switching. The displayed image has the important advantage of a wide viewing angle. In basic IPS mode LCDs, the pixel electrodes and common electrodes are each comb-shaped. The electric field of these LCDs in a driven state is along a certain direction. When the displayed image is viewed at different oblique angles, an observer notices a quite large color shift.
Referring to FIG. 5, this is a schematic plan representation of a pixel area P of an IPS liquid crystal display as disclosed by U.S. Pat. No. 6,459,465 issued on Oct. 1, 2002. The pixel area P comprises a gate line 113 arranged in a first direction, a data line 115 and a common line 135 both arranged in a second direction orthogonal to the first direction, a TFT (thin film transistor) 120 positioned at an intersection of the data line 115 and the gate line 113, a pixel electrode 131, and a common electrode 133. The TFT 120 has a gate electrode 121, a source electrode 123 and a drain electrode 125, which are connected with the gate line 113, the data line 115 and the pixel electrode 131 respectively. The pixel electrode 131 and the common electrode 133 are spaced apart from each other. The pixel and common electrodes 131, 133 are each comb-shaped, with the teeth thereof being zigzagged. Portions of the teeth of the pixel and common electrodes 131, 133 that are parallel to each other in a first direction form a first sub-electrode group. Portions of the teeth of the pixel and common electrodes 131, 133 that are parallel to each other in a second direction form a second sub-electrode group.
When a voltage is applied, because the pixel and common electrodes 131, 133 have zigzagged structures, the electric field (not shown) generated is mainly along two directions. Turning to FIG. 6, the upper portion thereof shows part of the first sub-electrode group, and the lower portion thereof shows part of the second sub-electrode group. The liquid crystal molecules 130 in the upper and lower portions have different orientations, and the LCD exhibits a two-domain display effect. When viewing the LCD display from any oblique angle, the color shifts generated by the two domains counteract, and thus the overall color shift of the display is small.
However, at junctions of the first and second sub-electrode groups, the electric field is abnormal, and the liquid crystal molecules thereat cannot be driven properly. In other words, a disclination of the liquid crystal molecules is generated at the bends of the teeth of the pixel and common electrodes 131, 133. Light thereat cannot transmit properly, and the contrast ratio of the pixel area is lowered. Furthermore, the two-domain electrode configuration of the LCD inherently limits the display thereof. Equally good visual performance at various different viewing angles cannot be attained.
It is desired to provide a multi-domain IPS liquid crystal display which overcomes the above-described deficiencies.